The possibility that the action of vasopressin on water movement is mediated by an effect on a mechanism involving microtubules and microfilaments is being explored. The effects of agents which act on microtubules (colchicine, vinblastine, vincristine and podophyllotoxin) and microfilaments (cytochalasin B) on the response to vasopressin and to cyclic AMP are being investigated in the isolated toad bladder. Assay, isolation and characterization of microtubule protein in the bladder epithelial cells is being attempted. Electron microscopic studies of the incidence and distribution of microtubules and microfilaments within the epithelial cells are being performed.